TCF/LEFs and Wnt signaling in the nucleus.

Abstract

T-cell factor/lymphoid enhancer factor (TCF/LEF) transcription factors are the major end point mediators of Wnt/Wingless signaling throughout metazoans. TCF/LEFs are multifunctional proteins that use their sequence-specific DNA-binding and context-dependent interactions to specify which genes will be regulated by Wnts. Much of the work to define their actions has focused on their ability to repress target gene expression when Wnt signals are absent and to recruit β-catenin to target genes for activation when Wnts are present. Recent advances have highlighted how these on/off actions are regulated by Wnt signals and stabilized β-catenin. In contrast to invertebrates, which typically contain one TCF/LEF protein that can both activate and repress Wnt targets, gene duplication and isoform complexity of the family in vertebrates have led to specialization, in which individual TCF/LEF isoforms have distinct activities.

Hallmarks of the TCF/LEF family. (A) Schematic of TCF domains, showing the location of five conserved domains found in this family, the amino-terminal β-catenin-binding domain, the Groucho binding sequence (GBS), the high-mobility group (HMG) domain followed by a nuclear localization signal (basic tail), and the C clamp. The specific TCF shown is from the sea urchin Strongylocentrotus purpuratus. (B) Alignments of the five domains from six invertebrate TCFs and the four human family members. Consensus residues are highlighted in dark gray with conservative changes in light gray. The extent of conservation is far greater in the HMG, basic tail, and C-clamp domains compared with the β-catenin-binding domain and GBS. Three residues in the β-catenin-binding domain that contact β-catenin and are essential for interaction are marked with asterisks (; ). A GBS could not be identified in TCF/LEFs from porifera and cnidarians and some flatworms, whereas a C clamp is found in nearly all invertebrate TCFs (it is not found in some flatworm TCF/LEFs) and in some vertebrate TCF1 and TCF4 isoforms. The sequences shown are from the sponge Amphimedon queenslandica (gene bank accession number ADO16566.1), the cnidarian Hydra magnipapillata (XP_002159974.1), the parasitic flatworm Schistosoma mansoni (XP_002572116.1), the nematode Caenorhabditis elegans (NP_491053.3), the dipteran Drosophila melanogaster (NP_726522), the sea urchin Strongylocentrotus purpuratus (NP_999640) and human TCF1E (EAW62279.1), TCF4E (CAB97213.1), and LEF1 (NP_001124185), and TCF3 (NP_112573.1).

High-mobility group DNA-binding domain (HMG DBD) and Helper sites of Drosophila and vertebrate TCF/LEFs. Sequence logos were constructed from TCF/Pangolin sites described in and from a collection of vertebrate Wnt-regulated enhancers (N Hoverter, ML Waterman, and KM Cadigan, unpubl.).

Regulation of TCF/LEFs in invertebrates and vertebrates. (A) Most invertebrates have one TCF/LEF gene producing a protein that can recruit corepressors to Wnt targets in the absence of signaling. (B) In vertebrates, TCF/LEFs are more specialized, with TCF3 often fulfilling this repressive role. (C) When high levels of β-catenin are found in the nucleus after Wnt signaling, it binds to invertebrate TCF, displacing (or inactivating) corepressors and recruiting coactivators through its amino- and carboxy-terminal transactivation domains. (D) For at least some vertebrate Wnt targets, a TCF/LEF exchange occurs, in which Wnt/β-catenin signaling promotes HIPK2-dependent phosphorylation of TCF3, causing it to leave Wnt target gene chromatin, allowing TCF1-β-catenin to occupy the WRE, facilitating transcriptional activation. See text for more information.

TCF/LEF isoform diversity. (A) Schematic of the LEF1 locus with two promoters. The first promoter produces an mRNA with an internal ribosome entry site in the 5′ UTR (encoded by exon1) (; ). A second promoter in intron2 produces a truncated, dominant–negative form of LEF1 missing the β-catenin-binding domain (green rectangle) (; ). Red and blue domains are the HMG and basic domains for DNA binding (refer to ). (B) Schematic of the TCF7 locus (which codes for TCF1). Similar to LEF1, the TCF7 locus codes for full-length and dominant–negative forms through the use of two promoters (). (C) The TCF7L2 locus (codes for TCF4) produces multiple truncated isoforms in addition to the full-length form. A putative intron1 promoter has been proposed that produces a dnTCF4 similar to dnLEF1 and dnTCF1 (). A third promoter in intron5 has been defined (). This promoter produces a dnTCF4 isoform because it is missing the β-catenin- binding domain. The promoter lies immediately downstream from a 92-kb genomic interval (yellow rectangle) that contains the strongest known risk alleles for type 2 diabetes (black asterisk; rs7903146 and rs12255372 []) as well as an alternative polyadenylation signal that might produce the previously discovered inhibitory form referred to as TCF4N (red asterisk) (; ).